Adeno-associated virus (AAV) is a nonpathogenic, helper dependent member of the parvovirus family. One of the identifying characteristics of this group is the encapsidation of a single-stranded DNA (ssDNA) genome. In the case of AAV, the separate plus or minus polarity strands are packaged with equal frequency, and either is infectious. At each end of the ssDNA genome, a palindromic terminal repeat (TR) structure base-pairs upon itself into a hairpin configuration. This serves as a primer for cellular DNA polymerase to synthesize the complementary strand after uncoating in the host cell. Adeno-associated virus generally requires a helper virus for a productive infection. Although adenovirus (Ad) usually serves this purpose, treatment of AAV infected cells with UV irradiation or hydroxyurea (HU) will also allow limited replication.
Recombinant AAV (rAAV) gene delivery vectors also package ssDNA of plus or minus polarity, and must rely on cellular replication factors for synthesis of the complementary strand. While it was initially expected that this step would be carried out spontaneously, by cellular DNA replication or repair pathways, this does not appear to be the case. Early work with rAAV vectors revealed that the ability to score marker gene expression was dramatically enhanced when cells were co-infected with adenovirus, or transiently pretreated with genotoxic agents. This enhancement correlated with the formation of duplex DNA from the single-stranded virion DNA (vDNA). Similar induction of rAAV vectors has been observed in vivo following treatment with Ad, ionizing radiation, or topoisomerase inhibitors. However, the effect was highly variable between different tissues and cell types. It has more recently been suggested that reannealing of complementary vDNA from separate infecting rAAV particles may be an important pathway for rAAV transduction.
The requirement for complementary-strand synthesis, or recruitment, is now considered to be a limiting factor in the efficiency of rAAV vectors. The transduction rate for rAAV in mouse liver has been estimated at approximately 5% of hepatocytes after portal vein infusion of 4.2×1010 particles. Subsequent experiments revealed that the rAAV vDNA had been taken up into the nuclei of virtually all of the liver hepatocytes, and that the transduction potential of these genomes could be rescued by co-infection with adenovirus. This is consistent with an earlier report of up to 25% of mouse hepatocytes transduced by 1010 particles of rAAV in the presence of co-infecting adenovirus. Expression from rAAV in liver tissue coincides with the formation of duplex DNA and the vDNA appears to be lost if not converted to duplex within 5-13 weeks. Further experiments suggest that a subpopulation of mouse hepatocytes is transiently permissive for rAAV transduction in vivo.
Accordingly, the present invention addresses a need in the art for improved parvovirus gene delivery vectors. In particular the present invention addresses the requirement for complementary strand synthesis by conventional AAV gene delivery vectors.